Plasma has long been employed to process substrates (e.g., wafers) into semiconductor products, such as integrated circuits. In many modern plasma processing systems, a substrate may be placed onto an RF chuck for plasma processing inside a plasma processing chamber. The RF chuck may be biased with an RF signal, using RF voltages in the range from tens to thousands of volts and RF frequencies in the range from tens of KHz to hundreds of MHz. Since the RF chuck also acts as a workpiece holder, proper control of the RF chuck temperature is an important consideration to ensure repeatable process results.
Generally speaking, the RF chuck's temperature is maintained by one or more electric heaters, which may be integrated or coupled with the RF chuck. Electrical power to the electric heater is typically obtained from line AC voltage via an appropriate control circuit to maintain the RF chuck at a desired temperature range. By way of example, the electric heater may be powered by DC, line frequency (e.g., 50/60 Hz AC) or KHz range AC power.
In this configuration, the DC/low frequency power needs to be coupled to the RF chuck assembly, which is also simultaneously subject to substantial levels of RF power either by stray coupling or by direct connection. To prevent an undesirable apparent RF short to ground, loss of RF power and high levels of signal interference, even damage via the electric heater power supply and/or control circuitry, RF isolation is required.
To facilitate discussion, FIG. 1 shows relevant portions of an example system that employs AC line (e.g., 50/60 Hz) voltages or DC voltages to power a heater or other load circuits at the RF hot or “high side”. Referring to FIG. 1, AC line voltages or DC voltages are supplied via leads 102 and 104 to RF filter circuit 106. RF filter circuit 106 is shown to be a single-channel (includes 2 wires for 1 complete circuit, to power 1 heater zone), dual-frequency filter and may include L-C circuits of a known design to present a high impedance to RF frequencies of interest (e.g., 2 MHz and 13.5 MHz) such that a relative RF short to ground via leads 102 and 104 and any attached circuitry, e.g. heater control/powering circuitry is effectively prevented. For illustration purposes, these RF frequencies are coupled to heater 114 via lead 116 as shown. In the example of FIG. 1, 114 is the load including the heater and high side control circuitry. On the other hand, 116 represents a leakage path, such as stray capacitance that would allow RF from the plasma or applied to the chuck to how back via the heater load, for example, RF filter 106 may have different designs and multiple stages to handle a wide range of discrete RF frequencies. The operation of RF filter 106 in its various implementations is basically known technology and will not be elaborated here.
Filter outputs 110 and 112 provides power to a load, e.g. heater, 114. A control circuit (not shown) may be coupled to leads 102 and 104 to turn on/off the input AC line voltages or DC voltages to control the temperature of an RF chuck, for example. The control may be performed in a proportional or in a binary on/off manner. Temperature sensing of the RF chuck may be employed as a feedback signal to the control circuit, for example.
When RF filters are employed to provide RF isolation in a high power, high RF frequency application, several drawbacks are encountered. The high RF frequency (e.g., in the MHz range) necessitates the use of large air core inductors in some designs, rendering the filtering circuit bulky. Furthermore, the goal is to maintain a sufficiently high input impedance (when viewed from the RF hot direction) across all the frequencies of interest. RF isolation design is complicated, however, by the filet that some plasma processing systems employ RF frequency tuning during processing. RF frequency tuning employs a range of frequencies during operation, thus making the RF isolation filter design significantly more challenging and complex due to the need to handle variable RF frequencies (and hence a wide range of RF impedances) and the desire to maintain system-to-system RF impedance and attenuation consistency. Even if an RF isolation filter design can handle a wide range of operating frequencies (fundamentals as well as their harmonics) and can be carefully matched to provide acceptable system-to-system uniformity with respect to RF isolation, care must still be taken to avoid high voltage discharge or arcing/breakdown and excessive heat generation or power dissipation may still be problematic. The design task is seriously complicated by the magnitude of the RF signal, which may be up to the range of thousands oh volts and up to the range of thou of watts.